Rounded and filleted prisms in OpenSCAD

OpenSCAD is great, but rounding tends to be slow, and creating fillets to join parts even slower. I've used a general purpose library in the past, but stunningly slow so ...

This uses OpenSCAD as a 3D library and ignores (mostly) the CSG aspect to create shapes that are hard with OpenSCAD primitives.

Uses the skin() module from list_comprehensions, but this needs stuff dropped from current scad_utils, so I've hauled skin() and what it needs into my source.

2019-02-27: Updated with more